Basic Parameter Calculation for Lithium Battery

Energy Density

Take NCM battery for example
Volume energy density (Wh / L) = battery capacity (mAh) × 3.6 (V) / (thickness (cm) * width (cm) * length (cm)) Weight energy density (Wh / KG) = battery capacity (mAh) × 3.6 (V) / battery weight

Battery discharge rate (C)

The discharge rate refers to the current value required to discharge its rated capacity (Q) within a specified time, which is numerically equal to a multiple of the battery rated capacity. The charge and discharge current (A) / rated capacity (Ah), the unit is generally C (short for C-rate), such as 0.5C, 1C, 5C, etc. For example, for a battery with a capacity of 24 Ah: Discharge with 48A, the discharge rate is 2C, in turn, 2C discharge, the discharge current is 48A, the discharge is completed in 0.5 hours; Charging with 12A, the charging rate is 0.5C, conversely, 0.5C charging, the charging current is 12A, the charging is completed in 2 hours; The charge and discharge rate of the battery determines how fast we can store a certain amount of energy in the battery, or how fast we can release the energy in the battery.

State of charge (%)

SOC, the full name is State of Charge, the state of charge, also called the remaining capacity, which represents the ratio of the remaining capacity after the battery is discharged to its fully charged capacity. Its value range is 0 ~ 1. When SOC = 0, it means that the battery is completely discharged. When SOC = 1, it means that the battery is fully charged. The battery management system (BMS) is mainly to manage the SOC and make estimates to ensure the efficient work of the battery, so it is the core of battery management. At present, SOC estimation mainly includes open-circuit voltage method, ampere-hour measurement method, artificial neural network method, Kalman filter method, etc., which we will explain in detail later.

Internal resistance

Internal resistance refers to the resistance that the current flows through the battery when the battery is working. Including ohmic internal resistance and polarization internal resistance, where: ohmic internal resistance includes electrode material, electrolyte, diaphragm resistance and resistance of various parts; polarization internal resistance includes electrochemical polarization resistance and concentration polarization resistance.

The following figure shows a battery discharge curve, the X-axis represents the discharge capacity, and the Y-axis represents the battery open-circuit voltage. The ideal discharge state of the battery is a black curve, and the red curve is the true state when the internal resistance of the battery is considered.

Internal resistance

The discharge current I and the battery’s internal resistance R, and the end-of-discharge voltage EDV. It should be pointed out that the battery internal resistance Rabat will gradually increase with the use of the battery. The unit of internal resistance is generally milliohm (mΩ). A battery with a large internal resistance has large internal power consumption and serious heat generation during charging and discharging. Discharge application. Therefore, the smaller the internal resistance, the better the battery life and rate performance. Generally, the measurement methods of battery internal resistance include AC and DC test methods.

Battery self-discharge

Refers to the phenomenon of voltage drop in the process of standing open, also known as the battery’s charge retention ability In general, battery self-discharge is mainly affected by manufacturing process, materials, and storage conditions. Self-discharge is divided into two types according to whether the capacity loss is reversible: capacity loss is reversible, which means that the capacity can be recovered after recharging; capacity loss is irreversible, which means that the capacity cannot be recovered. At present, there are many research theories on the causes of battery self-discharge, which can be summarized into physical reasons (storage environment, manufacturing process, materials, etc.) and chemical reasons (electrode instability in the electrolyte, internal chemical reactions occur, and active substances are consumed Etc.), self-discharge of the battery will directly reduce the capacity and storage performance of the battery.

Battery life

It can be classified by cycle life and calendar life. Cycle life refers to the number of times the battery can be cycled to charge and discharge. That is, under ideal temperature and humidity, charge and discharge at the rated charge and discharge current, and calculate the number of cycles experienced when the battery capacity decays to 80%.
Calendar life refers to the time span under which the battery reaches the end-of-life condition (capacity decay to 80%) under specific operating conditions. Calendar life is closely combined with specific use requirements, usually need to specify specific working conditions, environmental conditions, storage intervals, etc.
Cycle life is a theoretical parameter, and calendar life is more practical. However, the calculation of calendar life is complicated and takes a long time, so general battery manufacturers only give data on cycle life.

Battery pack consistency

Even after the battery cells of the same specification type are grouped, the performance of the battery pack in voltage, capacity, internal resistance, life, etc. is very different. When used in electric vehicles, the performance index often fails to reach the original level of single cells. After the cell is manufactured, due to process problems, the internal structure and material are not completely consistent, and there are certain performance differences. The initial inconsistencies accumulate with the continuous charge and discharge cycles of the battery during use, and the use environment in the battery pack is also different for every single cell, resulting in a greater difference in the state of each cell. It is gradually enlarged during use so that in some cases, the performance of some single cells is accelerated and decayed, and eventually leads to premature failure of the battery pack. It should be pointed out that the performance of the power battery pack depends on the performance of the battery cell, but it is by no means a simple accumulation of the performance of the battery cell. Due to the inconsistent performance of the single cell, when the power battery pack is repeatedly used on the electric vehicle, various problems occur and the life span is shortened. In addition to requiring strict control of the process and the consistency of single cells as much as possible during production and assembly, the industry generally uses a battery management system with a balancing function to control the consistency of batteries in the battery pack to extend the product ’s service life.

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